This invention relates generally to sensing devices, and more particularly to absolute position sensing devices with high sensing resolution.
Position sensors are used in a variety of devices to allow electrical systems to sense motion or position of moving objects and components. For example, one use of position sensors is in computer interface devices, which allow a user to provide input to a computer system to manipulate computer-generated objects and environments and to instruct the computer to perform tasks. Computer interface devices use position sensors to detect a user's input to the computer system. For interface devices such as joysticks and mice, position sensors determine the location of a user manipulandum within a workspace in particular degrees of freedom. The position of the manipulandum is used by the computer to control a cursor or otherwise manipulate a computer environment. Force feedback interface devices also use input from position sensors in the determination of an output force, which is then output on the manipulandum by motors or other actuators in the interface device. Position sensors are also used in a variety of other applications, such as sensing the position of a rotary knob. Traditional position sensors used in computer interface devices and other devices include analog potentiometers and digital encoders.
Sensors have a sensing resolution, which determines the amount of motion or displacement detectable by the sensor; for example, a larger sensing resolution allows the sensor to detect smaller increments of movement. One problem with existing position sensors is that resolution of the sensors is limited by cost considerations. For example, digital encoders can provide precise position readings but have a resolution limited by the spacing of encoder divisions (e.g., slots or marks) whose movement is detected by the sensor. The higher the resolution, the more closely spaced the encoder divisions must be, requiring high precision and greater cost to produce the sensor. This can present a problem for interface devices that require very high precision, such as force feedback interface devices, yet are sold as low-cost, high-volume consumer products.
Another type of position sensor is a capacitive sensor. These types of sensors typically detect changes in position by measuring capacitance between two relative-moving pieces or components, i.e. the charge of the capacitor is measured. These types of conventional capacitive sensors rely on charge measurement electronics which are cost prohibitive for high volume consumer products and typically suffer from poor signal to noise performance, and thus may not be suitable for many applications. Other types of capacitive sensors measure a frequency of an output signal from the sensor, and determine a position of moving components based on the sensed frequency. However, these types of sensors also suffer from high-cost components that read the output signal frequency.